Spherical solder reflow method

ABSTRACT

The present disclosure relates to methods of making solder balls having a uniform size. More particularly, the disclosure relates to improved solder ball formation processes that prevent or reduce bridging/merging of two or more solder balls during reflow. The processes of the instant disclosure are desirable because they do not require a sifting step to obtain uniformly-sized solder balls.

This application is a divisional of Ser. No. 13/030,594, filed Feb. 18,2011, now U.S. Pat. No. 8,162,203.

TECHNICAL FIELD

The present disclosure relates to methods of making solder balls havinga uniform size. More particularly, the disclosure relates to improvedsolder ball formation processes that prevent or reduce bridging of twoor more solder balls during injection molding processes. The solderballs produced by the instantly disclosed methods are used toelectrically connect integrated circuits.

BACKGROUND OF THE DISCLOSURE

Semiconductor chips can be mounted in a flip-chip configuration, whereinthe chips contain solder balls between integrated circuit (IC) devicesand chip carriers. The solder balls provide an electrical connection andbond between a chip contact location and a substrate contact location.It is important that the size of the solder balls are uniform in orderto ensure that all of the chip contact locations will be electricallyinterconnected to a corresponding substrate contact location.

Various technologies exist for depositing solder bumps on to IC devicesat the wafer level, including, for example, evaporation, electroplating,screen-printing, jetting, ball dropping, and Controlled Collapse ChipConnect New Process (i.e., C4NP). These deposition methods are oftenreferred to as solder wafer bumping or C4 wafer bumping. Originally,solder wafer bumping was accomplished by evaporating both theball-limiting metallurgy (BLM) and solder through mask holes in an arrayfashion onto the wafer surface. As the demand for higher I/O density,and lower cost of flip-chip interconnections has increased, however,other deposition methods have been developed. For example, ball dropmethods, used to make micro-size solder balls, have recently gainedattention in the industry because these methods allow for flexibility inthe solder alloys used and finer pitch applications. Indeed, in U.S.Pat. No. 6,517,602, Sato et al. disclose a method of forming solderballs by using a droplet spraying method. This method enables productionof micro-sized solder balls, but does not enable formation of solderballs having diameters less than 50 microns due to nozzle-sizelimitations. Furthermore, the process described by Sato et al. does notproduce uniform solder balls, and thus a secondary sorting process isneeded to obtain solder balls of the same size.

More recently, injection molded solder (IMS) processing has been used asa cost-effective solder wafer bumping method. While solder balls formedby IMS are usually more uniform in size than those produced by “balldrop” techniques, the uniformity of the solder balls can be lost if thesolder balls escape from the mold plate and are allowed to merge withone another after reflowing occurs. Therefore, it would be desirable todevelop an IMS process wherein merging or bridging of the solder ballsis reduced or prevented.

SUMMARY OF THE DISCLOSURE

The present disclosure provides IMS methods that reduce and/or preventbridging of two or more solder balls during reflow of the solder. TheIMS methods of the instant disclosure provide cost-effective processesof making two or more solder balls that are uniform in size.

A first aspect of the present disclosure is a method of preventing orreducing bridging of two or more solder balls during an injection moldedsoldering process comprising:

-   -   (A) obtaining a mold plate comprising at least two cavities on a        top side of the mold plate, and at least one non-recessed space        running between the at least two cavities, wherein the at least        one non-recessed space is co-planar to the top side of the mold        plate, and extends through an entire length of the mold plate;    -   (B) obtaining a coverplate comprising at least two recesses on        an underside of the coverplate, and at least one non-recessed        space running between the at least two recesses, wherein the at        least one non-recessed space is co-planer to the underside of        the coverplate, and extends through an entire length of the        coverplate;    -   (C) filling the at least two cavities with a molten solder;    -   (D) cooling the molten solder to a temperature that is below        about ½ of the solder's melting point to form a solidified        solder;    -   (E) forming a gap between a top of the mold plate and the        underside of the coverplate by positioning the at least two        recesses of the coverplate over a top of the at least two        cavities of the mold plate and aligning the recesses of the        coverplate, at the cavity scale, with the cavities of the mold        plate; and    -   (F) reflowing the solidified solder in an oxide-reducing        environment to form at least two solder balls,    -   wherein a height of the gap between the top of the mold plate        and the underside of the coverplate is about ⅓ to about ¾ of a        diameter of each of the at least two solder balls.

The height of the gap between the top of the mold plate and theunderside of the coverplate is typically about ⅓ to about ¾ of adiameter of each of the at least two solder balls, is more typicallyabout ½-⅔ of a diameter of each of the at least two solder balls, and ismost typically about ½ of a diameter of each of the at least two solderballs.

Reflow occurs in an oxide-reducing environment at temperatures above thesolder melting point, and can be accomplished using either liquid orgaseous flux. After reflow is complete, the coverplate is separated fromthe mold plate and the solder balls are then removed from the cavities.The shape of each of the cavities of the mold plate is typically round,hemispherical, pyramidal, cubic, hexahedron, octagonal, or cross-shaped.The mold plate is usually formed from a glass, metal, graphite, silicon,ceramic, or polymer material. Furthermore, the mold plate and/orcoverplate may comprise at least one gas vent channel that extendsthrough a middle of the non-recessed space running between the at leasttwo cavities and/or the at least two recesses. When present, the gasvent channel extends through the entire length of the mold plate and/orcoverplate.

A second aspect of the disclosure is a method of preventing or reducingbridging of two or more solder balls during an injection moldedsoldering process comprising:

-   -   (A) obtaining a mold plate comprising at least two cavities on a        top side of the mold plate, and at least one non-recessed space        running between the at least two cavities, wherein the at least        one non-recessed space is co-planar to the top side of the mold        plate, and extends through an entire length of the mold plate,        and wherein the at least two cavities have a uniform depth;    -   (B) obtaining a coverplate comprising at least two recesses on        an underside of the coverplate, and at least one non-recessed        space running between the at least two recesses, wherein the at        least one non-recessed space is co-planer to the underside of        the coverplate, and extends through an entire length of the        coverplate, and wherein the coverplate is a flux plate and a        depth of the at least two recesses is approximately equivalent        to the depth of the at least two cavities of the mold plate;    -   (C) filling the at least two cavities with a molten solder;    -   (D) cooling the molten solder to a temperature that is below        about ½ of the solder's melting point to faun a solidified        solder;    -   (E) filling the at least two recesses in the underside of the        coverplate with flux;    -   (F) fanning a solvent access gap by placing a spacer between the        top side of the mold plate and the underside of the coverplate,        and then aligning the recesses of the coverplate, at the cavity        scale, with the cavities of the mold plate; and    -   (G) reflowing the solidified solder to form at least two solder        balls,    -   wherein a height of the spacer is about ⅓ to about ¾ of a        diameter of each of the at least two solder balls.

The height of the spacer is typically about ⅓ to about ¾ of a diameterof each of the at least two solder balls, is more typically about ½-⅔ ofa diameter of each of the at least two solder balls, and is mosttypically about ½ of a diameter of each of the at least two solderballs.

Reflow occurs in an oxide-reducing environment at temperatures above thesolder melting point, and, in this aspect of the disclosure, isaccomplished using a liquid flux. After reflow is complete, thecoverplate is separated from the mold plate and the solder balls areremoved from the cavities. The shape of each of the cavities of the moldplate is typically round, hemispherical, pyramidal, cubic, hexahedron,octagonal, or cross-shaped. The mold plate is usually formed from aglass, metal, graphite, silicon, ceramic, or polymer material.Furthermore, the mold plate and/or coverplate may comprise at least onegas vent channel that extends through a middle of the non-recessed spacerunning between the at least two cavities and/or the at least tworecesses. When present, the gas vent channel extends through the entirelength of the mold plate and/or coverplate.

The spacer is typically placed either peripherally along the edges ofthe top side of the mold plate and the edges of the underside of thecoverplate, or along the non-recessed space separating the arrays ofcavities and recesses in the mold plate and coverplate, respectively. Ifthe spacer is placed peripherally along the edges of the mold plate andcoverplate, the spacer has breaks along its length so that the volumedefined by the facing surfaces of the mold and coverplates, as well asthe peripheral spacer, is never sealed. These breaks permit fluxsolvents to penetrate into the cavity area and remove flux residuestherefrom.

The second aspect of the present disclosure may further compriseremoving a flux residue formed in the at least two cavities of the moldplate after reflowing, wherein removing the flux residue comprisescontacting the flux residue with a solvent by infusing the solvent bycapillary action through the solvent access gap. The flux residue isremoved after reflow but before the coverplate is separated from themold plate. The reflowed solder balls remain arrayed in the cavity plateuntil after the flux residue is removed, and then they are gathered in acleaning tank or collection sieve through which the cleaning solutionwas discarded.

A third aspect of the present disclosure is a method of preventing orreducing bridging of two or more solder balls during an injection moldedsoldering process comprising:

-   -   (A) obtaining a mold plate comprising at least two cavities on a        top side of the mold plate, and at least one non-recessed space        running between the at least two cavities, wherein the at least        one non-recessed space is co-planar to the top side of the mold        plate, and extends through an entire length of the mold plate;    -   (B) obtaining a coverplate comprising at least one recess on an        underside of the coverplate, wherein the recess is sufficiently        large to cover more than one of the at least two cavities of the        mold plate and the non-recessed space between the at least two        cavities;    -   (C) filling the at least two cavities with a molten solder;    -   (D) cooling the molten solder to a temperature that is below        about ½ of the solder's melting point to form a solidified        solder;    -   (E) forming a gap between a top of the mold plate and the        underside of the coverplate by positioning the at least one        recess of the coverplate over a top of the at least two cavities        of the mold plate and aligning the recesses of the coverplate,        at the matrix scale, with the cavities of the mold plate; and    -   (E) reflowing the solidified solder to form at least two solder        balls, wherein a height of the gap between the top of the mold        plate and the underside of the coverplate is about ⅓ to about ¾        of a diameter of each of the at least two solder balls.

The height of the gap between the top of the mold plate and theunderside of the coverplate is typically about ⅓ to about ¾ of adiameter of each of the at least two solder balls, is more typicallyabout ½-⅔ of a diameter of each of the at least two solder balls, and ismost typically about ½ of a diameter of each of the at least two solderballs.

Reflow occurs in an oxide-reducing environment at temperatures above thesolder melting point, and can be accomplished using either liquid orgaseous flux. After reflow is complete, the coverplate is separated fromthe mold plate and the solder balls are then removed from the cavitiesand collected. The shape of each of the cavities of the mold plate istypically round, hemispherical, pyramidal, cubic, hexahedron, octagonal,or cross-shaped. The mold plate is usually formed from a glass, metal,graphite, silicon, ceramic, or polymer material. Furthermore, the moldplate may comprise at least one gas vent channel that extends through amiddle of the non-recessed space running between the at least twocavities. When present, the gas vent channel extends through the entirelength of the mold plate.

A fourth aspect of the disclosure is a method of preventing or reducingbridging of two or more solder balls during an injection moldedsoldering process comprising:

-   -   (A) obtaining a mold plate comprising at least two cavities on a        top side of the mold plate, and at least one non-recessed space        running between the at least two cavities, wherein the at least        one non-recessed space is co-planar to the top side of the mold        plate, and extends through an entire length of the mold plate,        and wherein the at least two cavities have a uniform depth;    -   (B) obtaining a coverplate comprising at least one recess on an        underside of the coverplate, wherein the recess is sufficiently        large to cover more than one of the at least two cavities of the        mold plate and the non-recessed space between the at least two        cavities and a depth of the at least one recess is approximately        equivalent to the depth of the at least two cavities of the mold        plate, and wherein the coverplate is a flux plate;    -   (C) filling the at least two cavities with a molten solder;    -   (D) cooling the molten solder to a temperature that is below        about ½ of the solder's melting point to form a solidified        solder;    -   (E) filling the at least one recess in the underside of the        coverplate with flux;    -   (F) forming a solvent access gap by placing a spacer between the        top side of the mold plate and the underside of the coverplate,        and then aligning the recess of the coverplate, at the matrix        scale, with the cavities of the mold plate; and    -   (G) reflowing the solidified solder to form at least two solder        balls,    -   wherein a height of the spacer is about ⅓ to about ¾ of a        diameter of each of the at least two solder balls.

The height of the spacer is typically about ⅓ to about ¾ of a diameterof each of the at least two solder balls, is more typically about ½-⅔ ofa diameter of each of the at least two solder balls, and is mosttypically about ½ of a diameter of each of the at least two solderballs.

Reflow occurs in an oxide-reducing environment at temperatures above thesolder melting point, and, in this aspect of the disclosure, isacomplished using a liquid flux. After reflow is complete, thecoverplate is separated from the mold plate and the solder balls areremoved from the cavities. The shape of each of the cavities of the moldplate is typically round, hemispherical, pyramidal, cubic, hexahedron,octagonal, or cross-shaped. The mold plate is usually formed from aglass, metal, graphite, silicon, ceramic, or polymer material.Furthermore, the mold plate may comprise at least one gas vent channelthat extends through a middle of the non-recessed space running betweenthe at least two cavities. When present, the gas vent channel extendsthrough the entire length of the mold plate.

The spacer is typically placed peripherally along the edges of the topside of the mold plate and the edges of the underside of the coverplate.The spacer has breaks along its length so that the volume defined by thefacing surfaces of the mold and coverplate, as well as the peripheralspacer, is never sealed. These breaks permit flux solvents to penetrateinto the cavity area and remove flux residues therefrom.

The fourth aspect of the present disclosure may further compriseremoving a flux residue formed in the at least two cavities of the moldplate after reflowing, wherein removing the flux residue comprisescontacting the flux residue with a solvent by infusing the solvent bycapillary action through the solvent access gap. The flux residue isremoved after reflow but before the coverplate is separated from themold plate. The reflowed solder balls remain arrayed in the cavity plateuntil after the flux residue is removed, and then they are gathered in acleaning tank or collection sieve through which the cleaning solutionwas discarded.

Yet a fifth aspect of the present disclosure relates to a method ofpreventing or reducing bridging of two or more solder balls during aninjection molded soldering process comprising:

-   -   (A) obtaining a mold plate comprising at least two cavities on a        top side of the mold plate and at least one non-recessed space        running between the at least two cavities, wherein the at least        one non-recessed space is co-planar to the top side of the mold        plate, and extends through an entire length of the mold plate;    -   (B) obtaining a high-temperature resist film;    -   (C) injecting a molten solder into the at least two cavities,        wherein the injection occurs in an environment having an oxygen        concentration which is higher than about 5 parts per million;    -   (D) cooling the molten solder to a temperature that is below        about ½ of the solder's melting point to form a solidified        solder;    -   (E) forming an oxide skin over a top of the solidified solder;    -   (F) dispensing flux over a top of the at least two cavities of        the mold plate;    -   (G) inserting a spacer over a top of the mold plate;    -   (H) placing a layer of the high-temperature resist film over a        top of the spacer, wherein the high-temperature resist film is        unreactive with the molten solder; and    -   (I) reflowing the solder to form at least two solder balls.

The height of the spacer is typically about ⅓ to about ¾ the diameter ofeach of the at least two solder balls, is more typically about ½ toabout ⅔ the diameter of each of the at least two solder balls, and ismost typically about ½ the diameter of each of the at least two solderballs.

After the layer of high-temperature resist film is placed over the topof the spacer, and before reflow, either a clamp or a coverplate may beused to fix the position of the high-temperature resist film in place.If a coverplate is used, the coverplate need not comprise any recesses.The high-temperature resist film is usually a polyimide film, apolytetrafluoroethylene film such as Teflon® film, or a polyphenyleneether film.

Reflow occurs in an oxide-reducing environment at temperatures above thesolder melting point, and, in this aspect of the disclosure, isaccomplished using a liquid flux. The shape of each of the cavities ofthe mold plate is typically round, hemispherical, pyramidal, cubic,hexahedron, octagonal, or cross-shaped. The mold plate is usually formedfrom a glass, metal, graphite, silicon, ceramic, or polymer material.

Furthermore, this aspect of the present disclosure may further comprise:

-   -   (A) removing a flux residue from an outside of the at least two        solder balls;    -   (B) removing the high-temperature resist film from the top of        the mold plate; and    -   (C) removing the at least two solder balls from the at least two        cavities of the mold plate.

The flux residue is created during reflow and may be removed by usingcapillary action to infuse a solvent through the space between the topside of the mold plate and the underside of the layer ofhigh-temperature resist film that is created by the spacer, andcontacting the flux residue with the solvent. In this aspect of thedisclosure, the flux residue is removed after reflow occurs but beforethe high-temperature resist film and/or coverplate/clamp is/areseparated from the mold plate. Thus, the solder balls are retained inthe cavities during removal of the flux residue.

Lastly, a sixth aspect of the present disclosure is a method ofpreventing or reducing bridging of two or more solder balls during aninjection molded soldering process comprising:

-   -   (A) obtaining a mold plate comprising at least two cavities on a        top side of the mold plate, and at least one non-recessed space        running between the at least two cavities, wherein the at least        one non-recessed space is co-planar to the top side of the mold        plate, and extends through an entire length of the mold plate;    -   (B) obtaining a high-temperature resist film;    -   (C) injecting a molten solder into the at least two cavities,        wherein the injection occurs in an N₂ environment having an        oxygen concentration of less than about 2 parts per million;    -   (D) cooling the molten solder to a temperature that is below        about ½ of the solder's melting point to form a solidified        solder;    -   (E) inserting a spacer over a top of the mold plate;    -   (F) placing a layer of a high-temperature resist film over a top        of the at least two cavities of the mold plate, wherein the        high-temperature resist film is unreactive with the molten        solder; and    -   (G) reflowing the solder to form at least two solder balls,        wherein the reflowing is performed in a formic acid vapor phase        flux.

The height of the spacer is typically about ⅓ to about ¾ the diameter ofeach of the at least two solder balls, is more typically about ½ toabout ⅔ the diameter of each of the at least two solder balls, and ismost typically about ½ the diameter of each of the at least two solderballs.

After the layer of high-temperature resist film is placed over the topof the spacer, and before reflow, either a clamp or a coverplate may beused to fix the position of the high-temperature resist film in place.If a coverplate is used, the coverplate need not comprise any recesses.The high-temperature resist film is usually a polyimide film, apolytetrafluoroethylene film such as Teflon® film, or a polyphenyleneether film.

Reflow occurs in an oxide-reducing environment at temperatures above thesolder melting point, and, in this aspect of the disclosure, isaccomplished using a gaseous flux. The shape of each of the cavities ofthe mold plate is typically round, hemispherical, pyramidal, cubic,hexahedron, octagonal, or cross-shaped. The mold plate is usually formedfrom a glass, metal, graphite, silicon, ceramic, or polymer material.

Furthermore, this aspect of the present disclosure may further comprise:

-   -   (A) removing the high-temperature resist film from the top of        the mold plate; and    -   (B) removing the at least two solder balls from the at least two        cavities of the mold plate.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 demonstrates a solder ball produced by an injection moldingsoldering (IMS) method, wherein merging of the solder balls occurredduring reflow.

FIG. 2 is a pictorial representation (through a side-view) of a moldplate used in the IMS methods of the present disclosure.

FIG. 3 is a pictorial representation (through a side-view) of the moldplate of the present disclosure after the molten solder has beeninjected into the at least two cavities.

FIG. 4 is a pictorial representation (through a side-view) of acoverplate used in some of the IMS methods of the present disclosure.The depicted coverplate is an example of a cavity-scale coverplate andcould be used in, for example, methods of the first and second aspectsof the present disclosure.

FIG. 5 is a pictorial representation (through a side-view) of the gapthat is formed between the top side of the mold plate and the undersideof the coverplate after the single recess of the coverplate ispositioned over the top of the at least two cavities of the mold plate,and the recess of the coverplate is aligned, at the matrix scale, withthe cavities of the mold plate.

FIG. 6 is a pictorial representation (through both top and side views)of the gas vent channel that may be present in the mold plates and/orcoverplates used in the IMS methods of the instant disclosure.

FIG. 7 is a pictorial representation (through a side-view) of the aspectof the present disclosure wherein the coverplate is a flux plate. FIG.7A shows the flux plate apparatus before reflow occurs, and FIG. 7Billustrates the flux plate apparatus after reflow is completed and theflux has been removed from the recesses of the flux plate.

FIG. 8 is a pictorial representation (through a side-view) of thespacer, which has solvent-entry openings, that is used to form a solventaccess gap between the top side of the mold plate and the underside ofthe flux plate/coverplate in some of the IMS methods of the instantdisclosure.

FIG. 9 is a pictorial representation (through a side-view) of anapparatus that may be used in the fifth and sixth aspects of thedisclosure that employ a high-temperature resist film and use a clamp tofix the position of the high-temperature resist film over the spacer.FIG. 9A shows the apparatus (through a side-view) after molten solderhas been injected into the cavities, and FIG. 9B illustrates theapparatus (through a side-view) after the solder has been reflowed toform solder balls.

FIG. 10 is a pictorial representation (through a side-view) of anapparatus that may be used in the fifth and sixth aspects of thedisclosure that employ a high-temperature resist film and uses acoverplate to fix the position of the high-temperature resist film overthe spacer. FIG. 10A shows the apparatus (through a side-view) aftermolten solder has been injected into the cavities, and FIG. 10Billustrates the apparatus (through a side-view) after the solder hasbeen reflowed to form solder balls.

DESCRIPTION OF BEST AND VARIOUS EMBODIMENTS OF DISCLOSURE

The present disclosure, which is directed to several injection moldingsoldering (IMS) methods of making solder balls, will now be described ingreater detail by referring to the drawings that accompany the presentapplication. Injection molding soldering refers to the method in whichmolten solder is injected into mold cavities and the solder is thenreflowed to form solder balls. It is noted that in the accompanyingdrawings, like reference numerals are used for describing like and/orcorresponding elements.

The IMS methods of the instant disclosure produce solder balls having auniform size by preventing and/or reducing bridging or merging of theballs. FIG. 1 illustrates an example of a solder ball 1 produced by anIMS method, wherein merging occurred and solder balls of various sizeswere produced. The solder balls made according to these methods must besifted after reflow in order to obtain several balls having uniformsize. The IMS methods of the present disclosure prevent and/or reducemerging of solder balls, and therefore eliminate the undesirable siftingstep required by many other IMS methods.

The term “solder ball,” as used in the present disclosure, refers to aball of solder material made according to the processes disclosedherein. The term solder ball includes material in any phase, includingmaterial that is in the solid or molten state. “Solder ball” also refersto a ball of solder material that is spherical or substantiallyspherical in shape. Solder balls of the instant disclosure usually havea diameter of about 25 microns to about 500 microns, and more typicallyfrom about 50 microns to about 150 microns.

Solder balls of the instant disclosure may be formed from soldermaterials comprising Sn, Pb, as well as alloys of Sn or Pb such as,SnPb, SnCu, SnAg, SnAgCu, SnCuBi, and SnAgCuBi. Typically, Sn63Pb37,Pb97Sn3 are used as leaded solders, and SnCu0.7, SnAg3.5, SnAg3.8Cu0.7are used as non-leaded solders. These materials are usually present in asubstantially pure form, wherein the material is essentially free ofother materials such as solder paste.

FIG. 2 illustrates an example of a mold plate 2 that may be utilized inthe IMS methods of the present disclosure. The mold plate 2 comprises atleast two cavities 3 in which the solder material (such as one or moreof the materials listed above) is deposited, and at least onenon-recessed space 4 running between the at least two cavities 3.

The at least two cavities 3 may be any shape, and are typically etchedinto the mold plate 2 using, for example, a precision etching process,such as an isotropic etching process. The diameter of each cavitytypically ranges from about 25 microns to about 500 microns, and moretypically ranges from about 50 microns to about 150 microns. Dependingon the etching process used to form the cavities, the depth to diameteraspect ratio of the cavities can, for example, range from about ¼ toabout ½.

The arrangement of cavities 3 in the mold plate 2 is also not limited.For example, the cavities may be arranged in a manner that mirrors thearrangement of solder receiving pads on a final substrate or wafer.Alternatively, the solder balls may be formed in a differentconfiguration (e.g., a more densely packed configuration) than that inwhich they are finally arrayed. The cavities may also be arranged in anarray in which they are evenly spaced in one or more dimensions (e.g., atwo-dimensional array of evenly spaced cavities).

The mold plate 2 may be formed from materials such as glass, metal,graphite, silicon, and/or polymer materials such as polyimide sheets.For instance, when the cavities 3 are round or hemispherical in shape, aglass or metal mold plate or sheet is more typically used. Whenpyramidal cavities are used, a silicon mold plate is more typicallyused, and the pyramidal shape is usually produced by anisotropic etchingin <100> silicon wafers.

The at least one non-recessed space 4 is co-planar to the top side ofthe mold plate 2, and extends through an entire length of the mold plate2. As a result of the IMS process, the cavities 3 of the mold plate 2are filled with the molten solder material 5 as shown, for example, inFIG. 3. After the cavities 3 of the mold plate 2 are filled, the moltensolder material 5 is cooled to form a solidified solder material.

FIG. 4 demonstrates an example of a cavity-scale coverplate 6 that isused in the first and second aspects of the IMS methods of the instantdisclosure. The cavity-scale coverplates 6 of the instant disclosurecomprise at least two recesses 7 which each cover one cavity 3 of themold plate 2. The coverplates further comprise at least one non-recessedspace 8 that runs between the at least two recesses 7.

When a coverplate, such as the coverplate 6, is employed in the methodsof the instant disclosure, a gap A is formed between a top of the moldplate 2 and the underside of the coverplate. The gap is formed, forinstance, by positioning the at least two recesses 7 of the coverplate 6over a top of the at least two cavities 3 of the mold plate 2 andaligning the recesses 7 of the coverplate 6, at the cavity scale, withthe cavities 3 of the mold plate 2. As demonstrated in FIG. 5, whichshows a matrix-scale coverplate, the height of the gap A issubstantially smaller than the diameter B of each of the solder ballsformed by the IMS methods of the instant disclosure, thus making itimpossible for the reflowed solder balls to bridge through gap A. Theheight of the gap A is typically about ⅓ to about ¾ of a diameter B ofeach of the at least two solder balls, is more typically about ½-⅔ of adiameter B of each of the at least two solder balls, and is mosttypically about ½ of a diameter B of each of the at least two solderballs.

The coverplates and/or mold plates of the instant disclosure may alsocomprise a gas vent channel 9 that extends through a middle of thenon-recessed space 4 running between the at least two cavities 3 and/orthe at least two recesses 7. FIG. 6 is a pictorial representation ofboth a top and side view of one aspect of the present disclosure,wherein the gas vent channel 9 extends through a middle of thenon-recessed space 4 that runs between arrays of cavities 3 in the moldplate 2. The gas vent channel 9 extends through the entire length of themold plate 2, enabling venting of any gas that may form during thesolder reflow using flux. Gas vent channels have a width that istypically 2-10 times the diameter of each of the cavities of the moldplate and/or the diameter of each of the recesses of the coverplate, andtypically have a depth that is 5-15 times the depth of each of thecavities of the mold plate and/or the depth of each of the recesses ofthe coverplate.

FIG. 7 demonstrates a further aspect of the present disclosure, whereinthe coverplate is a flux plate 10 comprising recesses 7 having a depth Cthat is approximately equal to a depth D of the cavities 3 of the moldplate. In this aspect of the disclosure, a spacer 11 is used to form asolvent access gap 12 between the top side of the mold plate and theunderside of the flux plate 10. The height E of the solvent access gap12 is typically about ⅓ to about ¾ of a diameter B of each of the atleast two solder balls, is more typically about ½-⅔ of a diameter B ofeach of the at least two solder balls, and is most typically about ½ ofa diameter B of each of the at least two solder balls. Before thesolvent access gap 12 is formed, the recesses 7 are filled with flux 13.

FIG. 8 provides further detail of the spacer 11 that is used to form asolvent access gap between the top side of the mold plate and theunderside of the flux plate or coverplate. The spacer 11 also providessolvent-entry openings 12.5. The spacer 11 has a height (2x) that isapproximately twice as large as the height (x) of the solvent-entryopenings 12.5. The spacers 11 permit entry of the flux solvents into thesolvent access gap 12, and, in smaller assemblies, may be positionedaround the periphery of the entire mold, between the top of the mold andthe bottom of the coverplate; or between cavity arrays in largerassemblies.

The fifth and sixth aspects of the IMS methods of the instant disclosureemploy a high temperature resist film 14, shown, for example, in FIG. 9,that is placed over the cavities of the mold plate before reflow occurs.In one aspect of the disclosure, the molten solder 5 is injected intothe at least two cavities 3 in an N₂ environment having an oxygenconcentration of less than about 2 parts per million as illustrated inFIG. 9A. After the molten solder 5 is injected, the high-temperatureresist film 14 is placed over the top of the cavities 3 and spacer 11 ofthe mold plate 2. The molten solder 5 is then cooled to form asolidified solder. The resulting solidified solder does not have anoxide skin over the top of it. After the molten solder is cooled, eithera clamp 15 or a coverplate 16 may be used to fix the position of thehigh-temperature resist film 14 on the spacer 11 during reflow. FIG. 9shows an apparatus of the instant disclosure wherein a clamp 15 is usedto secure the position of the high-temperature resist film 14.Furthermore, FIG. 9B demonstrates the apparatus of the instantdisclosure after reflow has occurred and solder balls 1 have beenformed. FIG. 10 shows the apparatus of the instant disclosure wherein acoverplate 16 is used to secure the position of the high-temperatureresist film 14. FIG. 10A shows the apparatus after the molten solder 5has been injected into the at least two cavities 3, and FIG. 10B showsthe apparatus after reflow has occurred and solder balls 1 have beenformed.

Next, the solidified solder is reflowed in a formic acid vapor phaseflux, or other oxide-reducing environment, to form solder balls. Forinstance, if the solder material used is a higher temperature Pb97Sn3alloy, then reflowing in hydrogen is possible because the environmenthas oxide-reducing properties above about 300° C. On the other hand, ifthe solder material is an Sn63Pb37 alloy, such as standard Sn63Pb37 witha melting temperature of about 183° C., or a lead-free alloy such asSnCu0.7, such as a common SnCu0.7 alloy with a melting temperature ofabout 227° C., then liquid flux or formic acid vapor environments may beused for reflow.

The term “high temperature resist film,” as used herein, usually refersto a polyimide film, a polytetrafluoroethylene film such as Teflon®film, or a polyphenylene ether film that does not react with the moltensolder. Other high temperature resist films that are unreactive withmolten solder, however, are also contemplated by the instant disclosure.

The temperature of the solder material during various stages of theprocesses of the instant disclosure depends on the specific soldermaterial or materials used. For example, standard eutectic SnPb solderhas a melting point of about 183° C., a lead-free alloy such as standardSnCu has a melting point of about 227° C., and standard high lead PbSn3has a melting point of about 323° C. Processing temperatures aretypically kept about 20° C. above the melting point for both cavityfilling as well as reflow.

At least one aspect of the present disclosure may overcome at least onedeficiency associated with prior injection molding soldering processes.In particular, the methods of the instant disclosure provide uniformlysized solder balls without requiring a sifting step after reflow. Inaddition, the solder balls of the instant disclosure, which are madewithout the use of solder paste, have relatively few voids and, as aresult, may be expected to have better overall mechanical propertiesthan solder balls produced using solder paste.

Still other objects and advantages of the present disclosure will becomereadily apparent by those skilled in the art from the preceding detaileddescription, wherein it is shown and described in preferred embodiments,simply by way of illustration of the best mode contemplated. As will berealized the disclosure is capable of other and different embodiments,and its several details are capable of modifications in various obviousrespects, without departing from the disclosure. Accordingly, thedescription is to be regarded as illustrative in nature and not asrestrictive.

The term “comprising” (and its grammatical variations) as used herein isused in the inclusive sense of “having” or “including” and not in theexclusive sense of “consisting only of.” The term “consistingessentially of” as used herein is intended to refer to including thatwhich is explicitly recited along with what does not materially affectthe basic and novel characteristics of that recited or specified. Theterms “a” and “the” as used herein are understood to encompass theplural as well as the singular.

1. A method of preventing or reducing bridging of two or more solderballs during an injection molded soldering process comprising: (A)obtaining a mold plate comprising at least two cavities on a top side ofthe mold plate and at least one non-recessed space running between theat least two cavities, wherein the at least one non-recessed space isco-planar to the top side of the mold plate, and extends through anentire length of the mold plate; (B) obtaining a high-temperature resistfilm; (C) injecting a molten solder into the at least two cavities,wherein the injection occurs in an environment having an oxygenconcentration which is higher than about 5 parts per million; (D)cooling the molten solder to a temperature that is below about ½ of thesolder's melting point to form a solidified solder; (E) forming an oxideskin over a top of the solidified solder; (F) dispensing flux over a topof the at least two cavities of the mold plate; (G) inserting a spacerover a top of the mold plate; (H) placing a layer of a high-temperatureresist film over a top of the spacer, wherein the high-temperatureresist film is unreactive with the molten solder; and (I) reflowing thesolder to form at least two solder balls, wherein a height of the spaceris about ⅓ to about ¾ of a diameter of each of the at least two solderballs.
 2. The method of claim 1 further comprising: (A) removing a fluxresidue from an outside of the at least two solder balls; (B) removingthe high-temperature resist film from the top of the mold plate; and (C)removing the at least two solder balls from the at least two cavities ofthe mold plate.
 3. The method of claim 1, wherein each of the at leasttwo cavities of the mold plate has a round, hemispherical, pyramidal,cubic, hexahedron, or cross shape.
 4. The method of claim 1, wherein themold plate comprises at least one of: a glass, metal, graphite, silicon,ceramic or polymer material.
 5. The method of claim 1, wherein thehigh-temperature resist film is a polyimide film, apolytetrafluoroethylene film, or a polyphenylene ether film.
 6. A methodof preventing or reducing bridging of two or more solder balls during aninjection molded soldering process comprising: (A) obtaining a moldplate comprising at least two cavities on a top side of the mold plateand at least one non-recessed space running between the at least twocavities, wherein the at least one non-recessed space is co-planar tothe top side of the mold plate, and extends through an entire length ofthe mold plate; (B) obtaining a high-temperature resist film; (C)injecting a molten solder into the at least two cavities, wherein theinjection occurs in an N₂ environment having an oxygen concentration ofless than about 2 parts per million; (D) cooling the molten solder to atemperature that is below about ½ of the solder's melting point to forma solidified solder; (E) inserting a spacer over a top of the moldplate; (F) placing a layer of a high-temperature resist film over a topof the at least two cavities of the mold plate, wherein thehigh-temperature resist film is unreactive with the molten solder; and(G) reflowing the solder to form at least two solder balls, wherein thereflowing is performed in a formic acid vapor phase flux, and wherein aheight of the spacer is about ⅓ to about ¾ of a diameter of each of theat least two solder balls.
 7. The method of claim 6, wherein each of theat least two cavities of the mold plate have a round, hemispherical,pyramidal, cubic, hexahedron, or cross shape.
 8. The method of claim 6,wherein the mold plate comprises at least one of: a glass, metal,graphite, silicon, ceramic, or polymer material.
 9. The method of claim6, wherein the high-temperature resist film is polyimide film, apolytetrafluoroethylene film, or a polyphenylene ether film.